.NET Tests .config loader

ConfigFileContext.cs

using System;
using System.Configuration;
using System.Linq;
using System.Reflection;

namespace {MyProject}_Tests
{
    public abstract class ConfigFileContext : IDisposable
    {
        public static ConfigFileContext Change(string path)
        {
            return new ChangeConfigFileContext(path);
        }

        public abstract void Dispose();

        private class ChangeConfigFileContext : ConfigFileContext
        {
            private readonly string oldConfig =
                AppDomain.CurrentDomain.GetData("APP_CONFIG_FILE").ToString();

            private bool disposedValue;

            public ChangeConfigFileContext(string path)
            {
                AppDomain.CurrentDomain.SetData("APP_CONFIG_FILE", path);
                ResetConfigMechanism();
            }

            public override void Dispose()
            {
                if (!disposedValue)
                {
                    AppDomain.CurrentDomain.SetData("APP_CONFIG_FILE", oldConfig);
                    ResetConfigMechanism();


                    disposedValue = true;
                }
                GC.SuppressFinalize(this);
            }

            private static void ResetConfigMechanism()
            {
                typeof(ConfigurationManager)
                    .GetField("s_initState", BindingFlags.NonPublic |
                                             BindingFlags.Static)
                    .SetValue(null, 0);

                typeof(ConfigurationManager)
                    .GetField("s_configSystem", BindingFlags.NonPublic |
                                                BindingFlags.Static)
                    .SetValue(null, null);

                typeof(ConfigurationManager)
                    .Assembly.GetTypes()
                    .Where(x => x.FullName ==
                                "System.Configuration.ClientConfigPaths")
                    .First()
                    .GetField("s_current", BindingFlags.NonPublic |
                                           BindingFlags.Static)
                    .SetValue(null, null);
            }
        }
    }
}

ConfigFileContextInitializer.cs

using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace {MyProject}_Tests
{
    [TestClass]
    public class ConfigFileContextInitializer
    {
        static ConfigFileContext config;

        [AssemblyInitialize]
        public static void AssemblyInitialize(TestContext ctx)
        { 
            config = ConfigFileContext.Change(@"..\..\..\{MyProject}\web.config");
            DatabaseConfig.Initialize();
        }

        [AssemblyCleanup]
        public static void AssemblyCleanup()
        {
            if (config != null)
                config.Dispose();
        }
    }
}

• Replace {MyProject} with your main project namespace
• Put these two files in test project
• Replace "{MyProject}\web.config" with valid path to config file (web.config or app.config)